Callbacks static analysis optimizations

ActiveSupport::Callbacks are used though out Rails so any micro-optimizations "may" go a long way.

At the moment, the callback's object is checked every time the callback is run, see Callback#evaluate_method. But we do know the type of this object when the callback object is created.

The idea is to have Callback#initialize generate a the "Callback#call" method (omg metaprogramming) that runs the conditionals :if/:unless then fires the callback.